Ingot mold hot top heat insulation board and method of preparing same

ABSTRACT

AN INGOT MOLD HOT TOP MADE OF LIGHT WEIGHT HEAT INSULATION BOARD, ESPECIALLY ADAPTED FOR USE IN REGULATING THE SOLIDIFICATION OF A MOLTEN MASS OF METAL IN A MOLD, IS PREPARED FROM A COMPOSITION COMPRISING PERLITE, CELLULOSE FILLER MATERIAL AND BINDER. AT LEAST ONE SURFACE OF   THE RESULTING MATERIAL IS TREATED WITH SILICA, OR OTHER REFRACTORY MATERIAL.

June 19, 1973 c. G. SPROULE. JR.. ETAL 3,740,240

INGOT MOLD HOT TOP HEAT INSULATION BOARD AND METHOD OF PREPARING SAMEFiled Oct. 7. 1970 I f L 4| Li"? I I )40 Fl G. 3. F G. 4.

INVENTORS CHARLES G. SPROULEJI HERBERT F. WAGNER, Jr.

BY 6 a {4%, C PM ATTORNEYS United States Patent U.S. Cl. Mfr-38.25 11Claims ABSTRACT OF THE DISCLOSURE An ingot mold hot top made of lightweight heat insulation board, especially adapted for use in regulatingthe solidification of a molten mass of metal in a mold, is prepared froma composition comprising perlite, cellulose filler material and binder.At least one surface of the resulting material is treated with silica,or other refractory material.

BACKGROUND OF THE INVENTION The present invention relates to ingot moldhot tops made of heat insulation material. More particularly, thepresent invention relates to heat insulation material adapted for use inconnection with ingot casting molds and ingot mold hot tops and to themethod of preparing said heat insulation material. Prior art hot topsare disclosed in Daley 3,166,807; Demaison 3,106,756; Davidson3,076,239; and Shephard, Jr., et a1. 3,006,046.

In the formation of steel or other metal ingots, molten metal is pouredinto an ingot mold, allowed to solidify, and thereafter removed fromprocessing by rolling, stamping, forging, and the like. As the moltenmetal becomes solidified in the ingot mold it shrinks. Without a feederhead or reservoir of molten metal, referred to in the industry as a hottop, to supply the ingot with additional metal during this period,shrinkage or contraction cavities known as pipe form in the ingot andrender the ingot partially or wholly unsatisfactory.

It is essential that solidification of molten metal in an ingot mold begradual; with the metal initially solidifying at the bottom of the moldand finally solidifying at the top of the mold. If very slow coolingoccurs and a hot top is employed, not only can the formation of pipe beavoided in the ingot itself, but impurities are permitted to rise to theportion of the ingot that is normally sheared or cropped. Thus, theprime function of the hot top is to supply a well of molten metal to thesolidifying ingot as the metal in that ingot cools and shrinks involume. To maintain the necessary reservoir of metal molten long enoughto supply the cooling ingot, the hot top must be uneconomicallyoversized, or possess excellent heat insulation characteristics, and/ orbe supplied with a source of heat.

Typically, hot tops consist of cast metal casings lined with relativelythick heat insulating refractory materials, such as refractory bricks,to protect the casing and reduce heat losses. Although such hot tops areknown to produce excellent results, they are beset with problems.Relatively thick heat insulating refractory material is often somewhatirregular in shape, making it difficult to obtain a tight fit of therefractory material within the hot top casing and maintain a tight fitover a wide temperature range, thereby permitting metal which is beingcast to flow into cracks along joints of the refractory material, whichcannot only result in a substantial waste of metal, but can also resultin damage to the hot top casing itself upon contact with molten metal.One of the most common problems of this nature is the difiiculty ofobtaining a smooth junction between the hot top and the ingot mold.Flaws of this type have caused a large amount of ingot top portions tobe scrapped.

Utilization of seamless hot top linings formed in situ has onlysubstituted one problem for another. The weight of refractory liningshas long been regarded as a distinct drawback and this is particularlytrue when relatively thick hot top linings are cast in :situ. Moreover,removal of a monolithic lining is especially difficult and expensive. Inorder to be economically practical, it has been necessary in the past toreuse not only the hot top but also the hot top lining. This has meantthat the refractory lining has had to be removed from both the metalcasing and the ingot without damage to the hot top casing, the ingot orthe lining.

In the past, the method of forming insulating hot top sideboards hasbeen by forming a slurry of fibrous material in a tank, pouring theslurry into a mold, and then employing expensive vacuum equipment toremove excessive moisture. The resulting green sideboard is then placedin a drying oven where it must be completely dried. This procedure hascaused manufacturing costs to be high. Accordingly a desideratum hasbeen a heat insulation material which can be easily formed into suitableboard shape and which has the following characteristics: good insulationwith small heat capacity, exact thickness, light weight, lowmanufacturing cost, and versatile utility.

This invention relates to low volume sinkheads and to hot-top apparatusfor the production of ingots with sinkheads of such character. Moreparticularly, the invention relates to the art of hot-topping whichinsures the continuous feeding of hot cast metal into the ingots, and isconcerned directly with the eificient and continuous feeding of such hotmetal by the use of smaller amounts of feed metal made possible bycontrolling the heat losses in said hot tops.

In particular this invention relates to hot tops, sideboards, andlinings and covers for sideboards and hot tops. In addition, it relatesto an inexpensive method of forming and producing these hot tops,sideboards, and linings and covers, which due to their operatingtemperature and characteristics, furnish a means for controlling therate of heat flow from the hot top, sideboards, or linings, so as thusto result in the necessity of a much smaller sinkhead and an increasedproduction of good products from the ingots obtained.

It is common practice in the production of killed steel ingots to placea suitable hot top, sideboards, and/0r covers on each ingot mold toconfine the shrinkage cavity to a sinkhead at the top of the ingot andthus minimize the amount of steel that must be discarded in the rollingmill in order to produce products. therefrom that are free from pipe andexcessive chemical segregation. The prime function of the hot top systemis therefore to supply a well of molten metal to the solidifying ingotas the metal in the ingot freezes and shrinks in volume. The hot topitself has a limited efi'iciency which is dependent upon a number offactors but it must be remembered that upon the final solidification ofthe ingot there is still metal in the hot top which cannot be used andmust therefore be discarded.

In the art today it is well recognized that hot top efficiency can beimproved by using refractory linings with normal porosities, such asfirebrick or castables, and by using high porosity refractories, such asinsulating firebrick, both of which will have a lower insulating factorand have a higher specific heat than the materials used herein.

It is therefore a prime feature of this invention to provide a preformedboard of suitable high temperature insulating materials which will havelower heat storage and lower heat transfer properties so as thereby toreduce and control heat losses. The invention also contemplates Ice amethod of prefabricating boards for hot top systems which will producemore efficient and smaller hot tops with resultant smaller volumesinkheads and thus result in a greater percentage of the ingot weightbeing convertible into sound products, with a decrease in the loss ofmetal in the sinkhead contained in the hot top. The method ofprefabricating these boards also makes it possible to vary the thicknessand the refractoriness of the boards to further control the flow of heattherethrough.

One object of the invention is to provide a hot top which is very lightin weight and very economical to make and use, and one which is soinexpensive that it may be discarded after one use. In addition it maybe so designed and prefabricated that it is simple to assemble and placein a hot top as a lining, used as sideboards, or used as a hot topitself. In this connection, it may be stated that the width of the shelfthat results from the use of this thin prefabricated board for hottopping ingots is kept to a minimum as the material thickness necessaryto control the heat is considerably less than the material thickness ofthe refractories used in the prior art. The end result is to insure goodproducts from the ingot on rolling, and a narrow shelf at the hot topjunction of the ingot is an aid to this desired result.

Another object of the invention is to provide a hot top of which therefractoriness of the prefabricated thin, light weight boards may becontrolled. This is advantageous in producing a sinkhead with relativelysmooth walls. The combination of a narrow shelf at the junction of theingot and the sinkhead, and a sinkhead with smooth walls makes itpossible to roll sound sinkhead metal into a satisfactory end product,thus realizing a greater amount of end product from a given size ingotand sinkhead. This invention also relates to a method of makingrefractory only the necessary thickness of the boards outside surfacesso as to keep them as thermally eflicient as possible.

In the prior art, the normal weight of firebrick and castables isapproximately 125 lb. to 135 lb. per cubic foot while the insulatingfirebrick runs from 45 lb. to 80 lb. per cubic foot. The normal weightof the insulating boards of this invention is from lb. to 14 lb. percubic foot and said boards furnish a means for producing lightereasier-to-handle hot tops at a much lower cost and still provide abetter heat control than is now possible from the use of firebrick,castables, or insulating firebrick.

Another know method of improving hot top efiiciency is through the useof exothermic materials in sideboards, hot tops, and linings and coversfor hot tops and sideboards. These exothermic materials are relativelyexpensive to use, so that their cost may outweight the value ofincreased ingot yield. They are also often produced on an individualbasis from chemically inconsistent byproducts which further increasesthe cost and difiiculty of their preparation as Well as causingvariations in the results obtained through their use. In addition, sincethe exothermic sideboards, hot tops, and linings and covers are ratherpoor insulators, much of the heat from the exothermic reaction iswasted. Therefore, there is a need for a material which is free from theabove disadvantages, but which is capable of preventing or minimizingthe formation of pipes or other detrimental cavities in the metalingots.

SUMMARY OF THE INVENTION An object of the present invention is toprovide an ingot mold hot top of high temperature heat insulatingmaterial.

Another object of the present invention is to provide a rigid, heatinsulating hot-top material which has low thermal conductivity at hightemperatures.

Still another object of this invention is to provide a preformed heatinsulating hot-top board which can etfectively and efiiciently be usedto control the shrinkage cavity in an ingot mold during thesolidification of molten metal in said mold.

Yet another object of the present invention is to provide a heatinsulation hot-top material which is relatively simple and inexpensiveto produce, but which can be formed into a light-weight board which hasuniform dimensions.

A further object of the present invention is to provide a rigid, heatinsulating hot-top material for use in lining molten metal contactingportions of ingot molds and/or hot tops which can be removed easily fromsuch molds and/or hot tops and economically discarded after a singleuse.

A still further object of the present invention is to provide a heatinsulation material which can efficiently be used as a hot top withoutthe necessity of employing a metal hot top casing.

Another object of the present invention is to provide a rigid, hightemperature heat insulating material with a heat-destructible binder andan optimum of refractoriness.

Another object of the present invention is to provide a rigid, heatinsulating material with a heat-destructible binder, an optimum ofrefractoriness and having low thermal conductivity at high temperatures.

Another object is to provide a preformed heat insulating material with aheat-destructible binder, an optimum of refractoriness having lowthermal conductivity at high temperatures and adapted to be placed in oron ingot molds to control the solidification of molten metal therein,thereby preventing or minimizing the formation of pipe or otherdetrimental cavities in the metal ingots.

Another object is to provide an ingot liner board with aheat-destructible binder and an optimum of refractoriness which isrelatively simple and inexpensive to produce, but which is highlyeffective in use.

A further object is to provide a rigid, heat-insulating material with aheat-destructible binder and optimum refractoriness for use in liningmolten-contacting portions of ingot molds and/or hot tops.

A still further object of this invention is to provide a preformedheat-insulating board with a heat-destructible binder and an optimumrefractoriness for metal ingot molds which is relatively inexpensive,both as to the cost of the components of the board and as to the cost ofits manufacture, the board providing good heat insulation for the moltenmetal within the ingot mold.

In accordance with the present invention a heat insulation material isprepared which finds particular application in connection with ingotcasting molds and ingot mold hot tops. The composition of such materialcomprises perlite, cellulose fillers and a binder. The surface of heatinsulating board prepared from this material is treated with silica byimpregnation or other suitable means. In particular, heat insulatingboard of the present invention comprises from about 60 to about percentby weight of perlite, from about 5 to about 10 percent by weight of aheat destructible binder, from about 15 to about 30 percent by weight ofcellulose filler material. This board is subsequently treated on atleast one surface with about 2 to about 50 percent by weight of silica.

BRIEF DESCRIPTION OF THE DRAWINGS Other and further objects, advantages,and features of the invention will be apparent to those skilled in theart from the following detailed description thereof, taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a sectional view of the upper portion of an ingot moldprovided with a hot top, the metal receiving cavity of the hot top beinglined with the novel heat insulating material of the present invention;

FIG. 2 is a sectional view of an ingot mold in which the novel heatinsulating material of the present invention serves as a hot top for areservoir of molten metal which feeds the pipe or shrinkage cavity as ittends to form in the solidifying ingot;

FIG. 3 is a sectional view of the top portion of an ingot mold havingthe novel heat insulating boards of this invention lining the upperportion of the mold cavity; and

FIG. 4 is a sectional view of the top portion of an ingot mold having alining composed of multiple heat insulating boards of the presentinvention. In addition, FIG. 4 illustrates the use of the novel heatinsulating boards of this invention as a cover for ingot molds duringsolidification of molten metal within the mold.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the presentinvention, heat insulating material is produced which is especially welladapted to be formed into boards for use in controlling thesolidification of molten metal. The composition of the heat insulatingmaterial comprises perlite, cellulose fillers and binder and may containother ingredients, such as clay. These materials are present in thefollowing amounts: from about 60 to about 80 percent by weight ofperlite, from about 15 to about 30 percent by weight of cellulose fillermaterial, from about 5 to about percent by weight of a heat destructiblebinder, such as asphalt binder, and from about 3 to about 6 percent byweight of clay, such as kaolin clay. A typical analysis of the inorganicmaterial present in the composition is shown by the following table:

Percent by weight Minimum Maximum As previously indicated, at least onesurface of the resulting heat insulating material is treated with silicain an amount between about 2 percent by Weight and 50 percent by weight,and preferably between about 5 percent by weight and 12 percent byweight.

The dried heat insulating board, i.e. board which is not green isimpregnated with silica to a depth up to inch employing any suitableprocedure. For example, according to one procedure the board can bedipped into a silica solution and the depth of surface treatmentregulated by the depth of the solution, concentration of silica in thesolution, and the like. In accordance with another procedure, the boardcan be sprayed with a silica solution. The silica solution can be simplya dispersion of silica in liquid medium, such as water, such as Du PontsLudox, a colloidal silica, submicroscopic particles of silica incolloidal suspension in water, or such as Monsantos Psyton. If desired,conventional wetting agents such as Du Ponts Alkanol WXN, can be addedto the silica solution for further control of the surface treatment. Bythus regulating the location of the surface treatment and the depth ofimpregnation the thermal efficiency of the board can be modified forvarying applications. As the amount of silica impregnation is increased,the thermal efiiciency of the board decreases. It has also been foundthat there is a definite relationship between the mass of the sinkheadand the depth of refractory surface treatment necessary to maintain theintegrity of the heat insulating board. For example, it has been foundthat boards of the present invention perform properly when used with asinkhead of about 200 pounds if they have a surface treatment of about Vinch in depth. On the other hand, it has been found that the same boardsneed a surface treatment of about A; inch in depth if used with asinkhead weighing about 4,000 pounds. After 6 the board has been treatedwith silica it is then dried at room temperature or slightly elevatedtemperatures in an oven prior to utilization.

Heat insulating material having the composition described above providesexcellent characteristics when used to control the rate ofsolidification of a mass of molten metal in an ingot. Due to the lowthermal conductivity of this material, the heat of the melt is notdissipated rapidly through the walls of the mold and/or hot top, but isretained in the molten metal. In this manner, there is provided areservoir of molten metal in the upper portion of a mold or hot topwhich feeds the pipe or shrinkage cavity as it tends to form in thesolidifying ingot, thereby preventing or at least minimizing theformation of pipe in the metal ingot. The heat insulating material ofthe above composition is substantially incombustible at the temperaturesencountered in the production of steel ingots.

The heat insulating board of the present invention not only results inincreased ingot yield and improved quality, but results in significantsavings in time and labor. Hot tops can be constructed from the heatinsulating board which weigh about A to ,4; of the weight of hot topspreviously used. Because of their light weight and durability, hot topsformed from the heat insulating board can be easily handled by one manwithout material handling equipment.

Another advantage of the heat insulating material of the presentinvention is the fact that boards of the material can be formedrelatively inexpensively by conventional procedures, such as thecommercial Fourdrinier papermaking machine. Since the heat insulatingboard is so inexpensive to make, it can be discarded after only one use.

Boards thus formed can be varied in thickness, but are normally preparedin a thickness ranging from /2 to 1 /2 inches. Even though it ispossible to prepare such boards in varying uniform thicknesses, it isoften advantageous to laminate a number of boards together when thickerboards of heat insulating material are desired. Due to the compositionof the heat insulating material, the light Weight boards formed fromsuch material are easily trimmed to any desired shape, even when theboards have been laminated together. The strength of the resultingboards is such that they are not susceptible to breakage in handling andthey will not change in shape or otherwise deteriorate in storage.

One very useful application for the heat insulating material of thepresent invention is in the use of such material in hot tops. Referringparticularly to FIG. 1, numeral 10 designates an ingot mold which isillustrated in phantom outline. A hot top casing 11, which is normallycomposed of cast iron, is centered between flanges 12 and 13 whichextend from and overlap the upper edge of ingot mold 10. The square hottop casing 11 is lined on all four sides with sidewalls 14 of heatinsulating board having a layer 14a of silica. Casing 11 and sidewalls14 are seated on refractory ring 15 also made of heat insulating boardwith a layer 15a of silica, so that casing 11 is completely protectedfrom the molten steel being poured into ingot mold 10. A metal wiperstrip or chill 15b may be provided for refractory ring 15, if desired.The hot top casing 11 is also provided with trunnions 16 and 17 whichcan be used in removing the casing from ingot mold 10 after ingot 18 hassolidified. The resulting shrinkage cavity is shown at 19.

Since the heat insulating board of the present invention is structurallyquite rigid and can be joined together by a variety of suitable means,such as nailing, it is possible to use the heat insulating board itselfwithout a hot top casing for the purpose of controlling thesolidification of molten metal in an ingot. As shown in FIG. 2, a hottop 20 composed solely of heat insulating board with a layer 20a ofsilica is seated in flange 21 of ingot mold 22, which is illustrated inphantom outline. When the molten metal in the mold solidifies and ingot23 is formed, shrinkage cavity 24 occurs in the hot top.

The heat insulating board of the present invention can also be employedin the ingot mold itself when pipe in the upper portion of the ingotmold is not objectionable. As shown in FIGS. 3 and 4, the heatinsulating board of the present invention can be used to line the upperportion of an ingot in order to reduce the amount of heat radiated fromthat portion and thereby maintain a reservoir of molten metal until theremainder of the ingot has been completely solidified. The size, shapeand location of the shrinkage cavity or pipe can thereby be regulated.

Referring specifically to FIG. 3, high temperature board with a layer30a of silica is shown lining the upper portion of ingot mold 31. Due tothe high temperature heat insulating property of board 39, a moltenmetal reservoir 32 is provided as ingot 33 solidifies in mold 31.

A similar embodiment showing the application of the high temperature"board is illustrated by FIG. 4, Where two layers of high temperatureboard and 41 are bonded together and line the upper portion of ingotmold 42. A silica layer 40a is provided to protect bonded boards 40, 41from contact with the molten metal 45. A third high temperature board43, having a bottom layer 43a of silica, is employed as a cover whilemolten metal 44 is solidifying to form ingot 45.

While the invention has been described in terms of the production ofheat insulating hot-top board, it will be understood that if desired theheat insulating material of the present invention can be formed into aone piece hot top. The light weight of the heat insulating materialpermits a unitary hot top to be easily handled by one man and avoids thenecessity of joining heat insulating boards together with a U-shapedclip, nails, bolts, a friction fit or any other suitable means. Incontrast, a conventional hot top for as small an ingot mold as 22 inchesby 22 inches by 80 inches often Weighs as much as 120 pounds and cannotbe handled by one man without the help of equipment.

By varying the thickness of the insulating board the flow of heattherethrough can be controlled and it is possible to produce moreefiioient and smaller hot tops with a resultant smaller volume sinkheadand permit a greater percentage of the ingot weight to be converted intosound products. Although the heat insulating board of the presentinvention is rigid even at thicknesses of less than /2 inch, it will beunderstood that the thickness of the heat insulating board decreases itis possible to attach wire mesh or some other suitable material to theheat insulating board to insure that the board will remain in apredetermined form under the conditions in which it is used. For certainapplications it is desirable to apply a layer of some other material,such as aluminum, to one side of the heat insulation board or in betweenheat insulation boards laminated together. It will also be understoodthat it is not essential for the surfaces of the heat insulating boardto be fiat and the board may 'be provided with one or more corrugatedsurfaces.

If desired, other materials can be included with the heat insulationmaterial. For example, fibrous materials, such as asbestos; mineralwool; alumina powder; and the like can be incorporated.

While it is convenient to form boards of the heat insulating material ona Fourdrinier machine, it is to be understood that such boards can beformed in other ways. For example, heat insulating boards can also beformed by casting a slurry of the heat insulating material in a mold ofsuitable shape and size. This alternative method of production is notpreferred since production costs tend to be higher than when the boardsare formed on a Fourdrinier machine. Regardless of the method ofproduction employed, the resulting board of heat insulating materialmust be treated with finely divided silica in the percentages indicatedabove. This impregnation of the board with silica can be done in anyconvenient manner such as spraying, dipping, and the like using a silicasolution. It is not essential that the board be impregnated on bothsides with silica since effective maintenance of the high temperatureintegrity of the product has been observed when the board is impregnatedwith silica on only one side. This fact is consistent with theobservation that greater heat integrity is obtained when the hightemperature material is impregnated with silica after formation of theheat insulating material into a board than by the mere incorporation ofsilica with the other ingredients in the composition used to prepare thehigh temperature heat insulation board.

The heat-insulating boards of the present invention are relativelyinexpensive to use for they are inexpensive both as to the materialsemployed and the cost of manufacture. Thus, boards of the compositionset forth above, due to the types and amounts of materials used, can bemade rigid, heat-insulating, with a heat-destructible binder and be madein large sheets in a continuous commercial operation on a conventionalFourdrinier type papermaking machine, and the large sheets cut intoboards of any desired shape and size to later be made as refractory asdesired. Therefore, it is not necessary to press or cast the boardsindividually, as with the boards used heretofore, for theheat-insulating boards of the present invention may be made in largequantities in a continuous commercial operation. Generally, in theproduction of these boards on a Fourdrinier, the individual componentsare formed into slurries, the slurries transferred to one or more stockchests and pumped to a machine chest from where the stock is flowed ontoa moving wire of the machine. Water is subsequently removed from thematerial on the wire to form a self-supporting sheet which is thenpressed, cut into predetermined lengths, and dried to form rigid sheets.The rigid sheets are then trimmed to form heat-insulating boards of adesired shape and size.

The boards thus formed usually have a thickness of from about /2 to 1%inches, plus or minus inch. Generally, boards of 1 inch thickness arepreferred for lining the upper portion of the mold cavity of ingotmolds. For some applications, a number of the boards may be laminatedtogether to form thicker boards, bodies or blocks of heat-insulatingmaterials. At this point the boards have a density of from about 9 lb.to 13 lb. per cubic foot, with densities in the range of 10 lb. to 12lb. per cubic foot being preferred.

From the foregoing it will be seen that this invention is well adaptedto obtain all of the ends and objects hereinabove set forth, togetherwith other advantages which are obvious and which are inherent to thesystem.

It will be understood that various modifications can be made withoutdeparting from the invention. Obvious modifications will occur to thoseskilled in the art and it will therefore be recognized that theinvention is not limited to the precise embodiments shown and described.

What is claimed is:

1. In a heat insulating hot-top board comprising perlite, cellulosefiller, and a binder of an asphalt type, the improvement comprisinghaving at least one surface of the board coated with about 2 to about50% by weight of silica.

2. The heat insulating board of claim 1, in which clay is present withthe perlite, cellulose fillers and binder.

3. The heat insulating board of claim 1, in which the binder is a heatdestructible binder.

4. The heat insulating board of claim 3, in which the heat destructiblebinder is an asphalt binder.

5. In a method of preparing a heat insulating hot-top board composed ofperlite, cellulose filler, and binder, the improvement comprisingtreating at least one surface of said heat insulating board with fromabout 2 to about 9 50% by weight of silica, and then drying the silicatreated board.

6. The method of claim 5, in which the heat insulating board is treatedwith silica by impregnating the board with a silica solution.

7. The method of claim 5, in which the heat insulating board is treatedwith silica by spraying the board with a silica solution.

8. In a heat insulating hot-top board comprising from about 60 to about80 percent by Weight of perlite, from about to about 10 percent byweight of asphalt binder and from about to about percent by weight ofcellulose fillers, the improvement comprising having at least onesurface of said board impregnated with from about 2 to about percent byweight of silica.

9. The heat insulating board of claim 8, in which the board isimpregnated with silica to a depth of up to /1 inch.

10. The heat insulating board of claim 8, in which the silica isconcentrated on only one surface of the board.

11. In an ingot mold hot-top of lightweight heat insulation article,comprising a plurality of heat insulating hottop boards arranged incontiguous relationship and adapted to be mounted inside an ingot moldand cover 10 interior surfaces thereof to protect said interior surfacesfrom contact by molten metal and to heat insulate said molten metal,said hot-top boards including perlite, cellulose filler and binder, theimprovement comprising having at least one surface of the boards coatedwith about 2 to about 50% by weight of silica.

References Cited UNITED STATES PATENTS LORENZO B. HAYES, PrimaryExaminer U.S. Cl. X.R.

